This invention relates to methods and apparatus for generating charged particles for size measurement by electric mobility and deposition on a wafer and in particular relates to particle deposition for wafer inspection, surface cleaning and as seed nuclei in semiconductor device fabrication.
Solid particles of an accurately known particle size deposited on a wafer are useful as standards of calibration for wafer surface inspecting equipment. Solid particles deposited on a wafer can also be used as artificial contaminants for testing wafer cleaning tools to determine their efficiency for particle removal from the wafer surface. In addition, particles deposited on a wafer can be used as seed nuclei for subsequent processing to form unique thin films with desired physical or chemical properties.
Aerosols containing solid or liquid particles suspended in a gas medium are useful for a variety of purposes. When the aerosol particles are electrically charged, the particle size can be measured by measuring its electrical mobility in the gas medium. Charged particles in an aerosol can also be deposited on a semiconductor wafer as artificial contaminants for a variety of purposes.
One application relates to wafer surface inspection by laser light scattering, electron microscopy and other methods for detecting the presence of particles on the wafer surface and for measuring the particle size. Calibration of such surface inspecting tools requires depositing solid particles of an accurately known particle size on the wafer for use as calibration standards. For such applications, polystyrene latex (PSL) spheres are usually used. The PSL spheres are generally dispersed in an aqueous medium and atomized by a pressurized gas source to form droplets. The droplets are then evaporated to allow the individual PSL spheres to appear as solid, spherical particles of PSL suspended in the atomizing gas, thus forming a PSL aerosol. Aerosol refers to a gas containing suspended particles. PSL aerosol, therefore, refers to an aerosol in which the suspended particles are polystyrene latex spheres. The PSL spheres are then deposited from the aerosol onto the wafer surface to produce standard wafers for calibration purposes.
Deposition of small particles from an aerosol by the usual mechanisms of gravitational settling or diffusion is generally too slow and not suitable for practical applications. For most applications, the rate of deposition needs to be increased. This can be accomplished by using charged particles in combination with an electric field to cause increased rate of deposition by the application of an electrical force on the charged particles.
Aerosols produced by atomization usually are not highly charged. A common method to increase the particle charge is to expose the aerosol to a source of ionizing radiation from a radioactive material. The high energy nuclear particles of alpha, beta and gamma rays emitted by material undergoing radioactive decay ionize the molecules of the gas to form molecular ions of both a positive and a negative polarity. These molecular ions then collide with the aerosol particles suspended in the gas to cause the particles to become charged. The resulting particle charge is usually bipolar, meaning that some particles are positively charged, and some are negatively charge. Since roughly equal concentrations of positively and negatively charged particles are created, the aerosol remains substantially neutral even though the individual particles are charged. As a result, exposing an aerosol to a source of ionizing radiation from a radioactive material is often referred to as a neutralization process even though the end result also includes the production of charged particles of both a positive and a negative electrical polarity.
The most common radioactive material used for aerosol neutralization includes polonium 210 and krypton 85. Both of these materials are widely used. Polonium 210 is an alpha emitter with a half life of 138 days through radioactive decay, while krypton 85 is a beta emitter with a half life of 10.3 years. The use of radioactive ionizers for aerosol neutralization and aerosol particle charging are described in References 1 and 2.
Because of health, environmental and security concerns, radioactive materials for research, commercial or industrial use are generally regulated by appropriate governmental agencies. These regulations are becoming increasingly more stringent making the use of a radioactive ionizer a less desirable method for gas ionization and particle charging for wafer deposition and other applications. A non-radioactive alternative is therefore needed.
Another application relates to the generation and deposition of solid particles on a wafer for use as artificial contaminants for wafer cleaning studies. For such applications, the particles are generally deposited on a wafer. The wafer is then scanned by a scanning surface inspecting tool to determine the number of particles deposited. The wafer is then subjected to cleaning by the wafer cleaning tool. Following cleaning, the wafer is scanned again to provide a new particle count. The difference in the initial and final particle count is the number of particles removed by the wafer cleaning tool. The percent of particle removal is then referred to as a cleaning efficiency. Using wafers artificially contaminated by particles, the particle removal efficiency of cleaning tools can be easily measured.
For wafer cleaning studies, various particle materials need to be used. Particle materials of the greatest interest include silicon, silicon dioxide, silicon nitride, tungsten, and copper, among others. Dry solid particles of a variety of materials and sizes, therefore, need to be deposited on a wafer to produce test wafers for wafer cleaning studies. Since different particle materials have different adhesion force characteristics when deposited on the wafer surface, it is important that the material of particles used for testing the wafer cleaning tool be similar to the material of real contaminant particles found on the wafer.
Another application is the generation and deposition of solid or liquid particles on a wafer to serve as seed nuclei for subsequent wafer processing by chemical vapor deposition, atomic layer deposition, and other thin-film deposition processes for semiconductor integrated circuit device fabrication. Formation of thin film by various film formation processes is facilitated by the presence of seed nuclei for film formation and growth. For such applications, dry solid particles of the desired material can be deposited on a wafer. Alternatively, small liquid particles can be deposited on the wafer which can then be reacted chemically with another material or thermally processed to produce the desired solid seed nuclei for such applications.
In all of these applications, the number of particles deposited on the wafer is generally quite small, when compared to the number of particles needed to cover the wafer surface completely. As such, this application differs from other methods of droplet deposition for thin film fabrication such as those described in U.S. Pat. No. 5,316,579. For depositing droplets to form thin films, the number of droplets deposited must be sufficiently large to provide complete surface coverage to produce a continuous thin film on the surface for subsequent processing to form a solid thin film with the desired physical and/or chemical properties. For the present application, the number of particles deposited is small and the deposited particles remain as discrete entities on the wafer surface rather than as a continuous film on the wafer surface.